The objective of this project is to clearly define macrophage functions during the expression of tumor immunity. Utilizing a syngeneic rat tumor system, and exploiting the ability to separate peritoneal macrophages into density-dependent subclasses which are functionally distinct, we will examine macrophage effector activities in normal, tumor immune, and tumor-bearing animals. This project provides the means by which any alterations in macrophage functions can be examined from the standpoint of the subclasses, where perhaps subtle and yet significant differences may be readily dectable.